Zac, on the Android Beginners mailing list, has posted directions for using the G1/ADP1 with no sim at all. (This will also allow you to activate it over wifi.) It is not a sim-unlock - that you will have to find elsewhere.
I know everyone wants to upgrade/trade/switch from the T-mobile G1 to the Android Development (Phone/Platform) 1. Well, I've got some news for you..
Courtesy dream_kill (who got a unit with an engineering bootloader as a warranty replacement - way to go Tmob, give it to a hacker..), cmonex and JesusFreke, we now have an engineering bootloader reflasher in the wild. All it takes is root (sorry guys, still no RC30 hacks.) Want to get it too? (Updated Version!)